Summary of Significant Accounting Policies Regulatory Assets Disclosure (Details) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2012
Regulatory Assets [Line Items]
Regulatory disallowance on insurance proceeds		$ 15,000
Regulatory Assets, Noncurrent		333,953	$ 368,908	$ 263,321
Regulatory Assets, Current		82,712	68,562	52,250
Deferred environmental charges					$ 95,000
After-tax regulatory disallowance on insurance proceeds		9,100
interest income from deferred environmental expenses		5,300
Unrealized Loss On Derivatives [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	[1]	3,540	3,515	551
Regulatory Assets, Current	[1]	21,949	29,889	5,520
Asset Recoverable Gas Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent		2,098	5,971	1,936
Regulatory Assets, Current		19,274	21,794	23,795
Pension and Other Postretirement Plans Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent		189,111	201,845	118,485
Pension Balancing [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	[2]	41,193	32,541	30,682
Income Tax Asset [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent		44,767	47,427	49,007
Environmental Restoration Costs [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	[2],[3]	37,443	58,859	51,861
Regulatory Assets, Current		12,364	0	0
Decoupling [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Current	[4]	19,391	2,219	11,847
Other Regulatory [Member]
Regulatory Assets [Line Items]
Regulatory Assets, Noncurrent	[4]	15,801	18,750	10,799
Regulatory Assets, Current	[4]	$ 9,734	$ 14,660	$ 11,088

[1] Unrealized gains or losses on derivatives are non-cash items and, therefore, do not earn a rate of return or a carrying charge. These amounts are recoverable through utility rates as part of the annual Purchased Gas Adjustment (PGA) mechanism when realized at settlement.

[2] The deferral of certain pension expenses above or below the amount set in rates was approved by the OPUC, with recovery of these deferred amounts through the implementation of a balancing account, which includes the expectation of lower net periodic benefit costs in future years. Deferred pension expense balances include accrued interest at the utility’s authorized rate of return, with the equity portion of interest income recognized when amounts are collected in rates.

[3] Includes cash paid, remaining liability, and interest, net of insurance reimbursement and amounts reclassified to utility plant for the water treatment station.

[4] These balances primarily consist of deferrals and amortizations under approved regulatory mechanisms. The accounts being amortized typically earn a rate of return or carrying charge.